The present invention relates to control circuits, e.g., furnace controls, and is more particularly concerned with a control circuit with a gas valve relay driver circuit which will actuate the gas valve when all components are in proper order, but will fail to actuate the gas valve otherwise. The invention is also concerned with a furnace control circuit that operates with fewer and less expensive parts than those of the prior art.
In a modern gas furnace, one or more gas burners inject a gas flame through a heat exchanger, and the combustion gases are drawn out the heat exchanger by means of an inducer blower, which exhausts the combustion gases to a vent or flue. A pressure sensor associated with the inducer actuates a pressure switch to indicate a pressure differential between the exhaust and intake of the inducer. The pressure switch provides an indication that the inducer is functioning properly.
An indoor air blower forces air from a comfort zone past the heat exchanger to draw heat from the combustion gases. The warmed air is then returned to the comfort zone. A temperature limit switch on the heat exchanger is normally closed, and opens if the heat exchanger exceeds a predetermined temperature. This limit switch serves as a check on proper air flow and functioning of the indoor air blower.
A thermostat located in the comfort zone closes when the room temperature drops below a predetermined setpoint, and thereby signals a call for heat. When a call for heat is detected, control and timing circuitry for the furnace actuates the inducer blower and then initiates an actuation sequence which actuates a gas valve relay so that current is supplied to a gas valve. This allows combustion gas to flow to the burners. At this time, igniters are actuated to light the burners, and the furnace begins to produce heat. An infrared detector, rectification or other mechanism is employed to ensure that there is flame when the gas valve is actuated. If no flame is present, another series switch interrupts the thermostat power and turns off the gas valve.
After the burners have been ignited for a predetermined time, the room air blower is powered up, and this creates a flow of warm air to the interior comfort zone.
Conventionally, 24 volt ac thermostat power is supplied through the series arrangement of the limit switch, thermostat, pressure switch, gas valve relay, and gas valve. Optionally, a pilot relay can actuate a line-powered gas valve relay.
As aforementioned, the limit switch, thermostat, and pressure switch are all disposed in series with the gas valve relay, so that no current can flow through the gas valve relay to actuate the gas valve, until the limit switch and pressure switch are both closed. This serves as a check that the room air blower and the inducer blower are functioning properly.
A safety problem can arise if any of the limit switch, pressure switch, or gas valve relay are for some reason locked into a closed condition. In those cases, the gas valve will continue to feed gas to the burners if the heat exchanger experiences overtemperature, or if the inducer fails to produce sufficient draft.
In a modern furnace control unit, such as is shown in Erdman U.S. Pat. No. 5,074,780, a microprocessor circuit has respective inputs connected to the series switches, with a separate microprocessor input coupled to the junction between each switch and the next successive switch or operating element. This means that, for each switch, the microprocessor requires a separate input circuit. As the cost of the microprocessor depends on the number of input circuits that are needed, the cost of the control circuit can become great. Also, a failure of any one of the input circuits can prevent the device from detecting a switch malfunction or failure condition.
Generally, whenever there is a call for heat, the controller should be able to check the conditions of the pressure switch and the gas valve relay before supplying current to the coil for the gas valve relay. This permits the control circuit to check for switch malfunction and indicate a service condition, if service or repair is required. In the conventional system, each switch has a connection to an associated input. As each switch goes from an open to a closed condition, the respective microprocessor input circuit goes from a low to a high level. Thus when there is a call for heat, the input circuit associated with the thermostat goes to a high, and the microprocessor then is alerted to turn on the inducer blower. This pressure switch closes after the inducer creates a pressure differential, and the associated microprocessor input goes from low to high. Then the gas valve relay is actuated, and the microprocessor input associated with the gas valve relay goes from low to high. This arrangement permits a positive check that the pressure switch and gas valve relay can both open and close.
The gas valve relay has an actuator coil with a driver circuit that is controlled by an output of the microprocessor. The gas valve relay driver circuit is typically designed so that if any of the driver components fail, the gas valve will not turn on. That is, the gas valve driver circuit must be constructed in such a way that the gas valve will not be turned on inadvertently. Clearly, this safety feature is necessary to keep the room space around the furnace from flooding with unignited gas, as that could present a danger of either suffocation or explosion.
One fail-safe gas valve driver circuit has been described in U.S. Pat. No. 5,085,574, granted Feb. 4, 1992, to Larry E. Wilson. In that arrangement, the driver circuit comprises a transistor or similar switching device with a collector load resistor coupled to a voltage source, the actuator coil for the gas valve relay, a capacitor tied in series between the collector of the transistor and the actuator coil, and a diode in parallel to the actuator coil. The furnace controller emits pulsating dc to the transistor to actuate the coil. The capacitor blocks straight dc from actuating the coil. If there is a failure, in any of these driver components either in the nature of an open or a short, the circuit will deny current through the actuator coil, and the gas valve relay will fail to close. While this circuit is rather simple and straightforward, it does have a few drawbacks. For one, the capacitor is disposed on the coil side of the transistor, so the capacitor must be capable of absorbing large amounts of current. Therefore, a large and costly electrolytic capacitor is used, typically 50 .mu.f. Because of the high levels of current needed for the actuator coil, the pulsating dc produces a significant ripple on the capacitor. This shortens the life of the capacitor, causing frequent failures of the capacitor component. Likewise, high currents also appear across the diode, which can cause diode failure.
This system is also blind to the actual switch condition of the gas valve relay, and does not prevent gas valve actuation in the event that the gas valve or the gas valve relay contactor is frozen into a closed condition.